1. Field of the Invention
The present invention relates to a shift control system for an automatic transmission of a vehicle and more particularly, to an improved shift control system for a vehicle automatic transmission which can prevent a shift shock of the automatic transmission.
2. Description of Related Art
Generally, a transmission increases or decreases rotating power and rotating speed of an engine according to a driving condition of a vehicle, and also functions in an off-position wherein power from the engine is not transmitted to the driving wheels for a certain period of time so as to allow the engine to warm-up. The transmission industry classifies a manual transmission by manual operation, and an automatic transmission by automatic operation.
The automatic transmission related to the present invention is convenient to operate, enables the vehicle to start smoothly, to increase and decrease the car speed, and is less likely to stall when driven by a novice driver. Therefore, this automatic transmission is a popular choice among drivers.
The automatic transmission has "D", "2", "L", "N", "P", and "R" operating ranges. In the "D" range the automatic transmission shifts in multiple speed stages according to the degree the accelerator pedal is depressed (or, alternatively the throttle opening degree). The "2" range is for driving on a long downward sloping road, and the "L" range is for driving on an extremely downward sloping and curving road.
In the "N" range, the automatic transmission does not transfer power from the engine to the driving wheels, and does not place a load on the engine. In the "P" range, the automatic transmission fixes an output shaft of the automatic transmission while the vehicle is parked, and in the "R" range, the automatic transmission causes the vehicle to operate in reverse.
In the "D" range, the vehicle starts in a 1-speed or 2-speed state without any relation to the opening degree of a throttle valve or depression of the accelerator pedal, and causes an upshift to the 2-speed or 3-speed state according to the car speed.
Also, if the throttle valve is gradually opened, a driving gear of the 1-speed, 2-speed, and 3-speed has an increasingly higher shift point thereof. If the car speed decreases, the downshift automatically occurs. In this 3-speed or 2-speed state of the "D" range, if the accelerator pedal is depressed completely in order to suddenly accelerate the car, the automatic transmission enters a kick down state by downshifting from the 3-speed to 2-speed or the 2-speed to 1-speed.
The automatic transmission is provided with a hydraulic control system to control shifting by automatically operating brake bands and clutches of the transmission's star gear unit (i.e., planetary gear unit) according to the car speed, the engine loading degree (i.e., throttle opening degree or depression of the accelerator pedal), and position of the shifting lever (i.e. the selected operating range of the automatic transmission).
The hydraulic control system is controlled by a transmission control unit (TCU), and a fluid load of a line pressure controlled by manually operated valves. The TCU implements an open loop control system. Therefore, the star gear unit located at a rear portion of a torque converter performs an upshift or a downshift under the control of this open loop system. The TCU outputs control signals to shift control solenoids in the hydraulic control system based on the operating state of the vehicle and engine therefor. In response to the operation of the shift control solenoids, a shift control valve supplies hydraulic fluid, and thus pressure, to a plurality of valve bodies. The valve bodies in turn regulate the application of hydraulic pressure to the front clutch, the rear clutch, the end clutch, and the kick down brake of the automatic transmission. Additionally, the TCU outputs a control signal to a pressure control solenoid which regulates the hydraulic pressure applied to the front clutch, the rear clutch, the end clutch, and the kick down brake band via the valve bodies.
In the "D" range, shifting between the 1-speed to 4-speed, or recently, the 1-speed to 5-speed is accomplished in multiple stages. In detail, a rear clutch and a one-way clutch operate in the 1-speed state, the rear clutch and a kick down brake band operate in the 2-speed state, a front clutch, an end clutch, and the rear clutch operate in the 3-speed state, and the end clutch and the kick down brake band operate in the 4-speed state.
Such conventional shift control systems, however, suffer from a number of problems. For example, shift shock occurs due to an incomplete return operation of a kick down piston in the kick down brake during the 3 to 2-speed downshift operation when downshifting from 4-speed to 1-speed and when the engine and the vehicle operate at a low speed. Additionally, shift shock occurs when shifting from the 3-speed to the 2-speed state since a large speed difference is created in a short period of time between the input and output shafts of the automatic transmission; particularly, when the engine provides a large amount of torque.